75 vette rpm
Thread Starter
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Joined: Aug 2013
Posts: 53
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From: meas az
75 vette rpm
what rpm do you see at 70 mph ? i believe my tach is out of whack seems to work right at idle but at highway speeds not right at 75 its showing 4200 the car is all stock 1975 350 4 speed 20k original miles . thanks
Thread Starter
Advanced
Joined: Aug 2013
Posts: 53
Likes: 0
From: meas az
yes i understand that . but it is all stock i was looking to see what some other stock 75 vette owners are seeing for rpm at highway speeds i have been told they came with 323 gears but not sure
Racer
Joined: Jan 2014
Posts: 334
Likes: 6
To calculate expected rpm, you need
1) your wheel-tire combination and
2) your differential ratio.
To keep it simple, ignore possible slippage in the drive train.
Go to a tire calculator like this one
http://tire-size-conversion.com/tire-size-calculator/
For my '75 coupe I have 235/60R15 tires so I get 773 revs/mile.
Look at your differential casing to get the ratio code (assuming the original ratio is still in there.) I have a 3.08.
So, at 60 mph, I get (773)(3.08)= around 2400 rpm.
At 70 mph, I get (70/60)(773)(3.08)= around 2800 rpm.
I have yet to fix my tach, but this fits with how my '75 sounds. If your car spins way faster, putting the car on stands and marking the drive shaft while turning the rear wheels will verify if the rear end was changed or not.
1) your wheel-tire combination and
2) your differential ratio.
To keep it simple, ignore possible slippage in the drive train.
Go to a tire calculator like this one
http://tire-size-conversion.com/tire-size-calculator/
For my '75 coupe I have 235/60R15 tires so I get 773 revs/mile.
Look at your differential casing to get the ratio code (assuming the original ratio is still in there.) I have a 3.08.
So, at 60 mph, I get (773)(3.08)= around 2400 rpm.
At 70 mph, I get (70/60)(773)(3.08)= around 2800 rpm.
I have yet to fix my tach, but this fits with how my '75 sounds. If your car spins way faster, putting the car on stands and marking the drive shaft while turning the rear wheels will verify if the rear end was changed or not.
Le Mans Master
Joined: Feb 2001
Posts: 6,317
Likes: 292
From: Ocala FL
Try this.
http://www.angelfire.com/fl/procrastination/rear.html
With a 3.08 rear, you will get (rule of thumb) roughly 1000 for 25 mph. So 4000 rpm is roughly 100 mph.
Speedo at 70
Speedo on 75
http://www.angelfire.com/fl/procrastination/rear.html
With a 3.08 rear, you will get (rule of thumb) roughly 1000 for 25 mph. So 4000 rpm is roughly 100 mph.
Speedo at 70
Speedo on 75
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
L82 had 3.55
There's something wrong with your speedo or tach or both. Calibrate the speedo first using a GPS.
Thread Starter
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Joined: Aug 2013
Posts: 53
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From: meas az
speedo is dead on with GPS. . Yes i know my tach is not reading right i was just wording how far off it was . thanks for the pics i frigged it was at least of by 1000 rpm . now how do i fix it . thanks
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